Town continued their excellent away record in 2014 by blasting away the Daggers with three first-half goals, two of them coming from teenager Ivan Toney.
It means that a point against Oxford United at Sixfields on Saturday will preserve Northampton's Football League status, regardless of Wycombe's result after the Chairboys lost a crucial relegation battle against Bristol Rovers at Adams Park.
Anything other than a Wycombe victory next weekend will also see the Cobblers safe.
Manager Chris Wilder spoke to BBC Radio Northampton after the game and said: "We couldn't have asked for anything more. They were three quality goals and we've had fantastic support."